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Janet Gray, Ph.D.
Janet Gray, Ph.D.

As author of our 2008 and 2010 State of the Evidence reports, Dr. Gray drives the science behind all our work.

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Pregnancy and Lactation

During pregnancy, a woman’s breasts experience rapid changes as they prepare for breast-feeding. Maturation of the breast through the final stages of lactation contributes to longer-term protective effects against breast cancer. However, exposure to toxic chemicals during this vulnerable period may contribute to increased risks of pregnancy-related breast cancer.

Human breasts do not fully mature until a woman’s first full-term pregnancy and period of breast-feeding. During this time of rapid cell growth and proliferation, breast cells have heightened sensitivity to the detrimental effects of many environmental exposures. Thus, exposure to toxic chemicals during pregnancy may contribute to increased risks of pregnancy-related breast cancer (Andersson, 2009). Nevertheless, the process of full maturation of the breast during pregnancy and lactation imparts a protective effect against developing the disease following menopause (Russo, 2008). 

For all racial groups, the younger a woman is when she has her first child, and the more children she has, the less risk she has for developing breast cancer after menopause (Kauppila, 2009). However, for black women, while younger age on first giving birth and larger family size protect against breast cancer after age 45, it increases risk for breast cancer before age 45 (Palmer, 2003; Steube, 2009). Overall, black women under 45 have higher rates of breast cancer than other racial groups (Bowen, 2006), and the research relating pregnancy to pre-menopausal breast cancer has not explored social, environmental or economic factors that may influence this racial difference in risk. Therefore it is difficult to tease apart which aspects of pregnancy — whether pregnancy-related risks for breast cancer, higher exposure to environmental chemicals during pregnancy or other social factors — determine pre-menopausal risk for the disease.

Pregnancy presents a significant stage in which breast cells divide rapidly and key changes occur in the breast to prepare for breast-feeding (Kauppila, 2009). As a result, pregnancy itself is a vulnerable period for the development of breast cancer. The overall rates of pregnancy-associated breast cancer are small, but pregnancy slightly increases risk for several years after delivery, beyond which longer-term protective effects emerge (Lyons, 2009). While pregnancy-related breast cancers are relatively rare, tumors that develop at this time can be more aggressive than later-life breast cancers (Mathelin, 2008).